In the LTE system, the UE in the connected state needs to obtain uplink synchronization and downlink synchronization with the base station before sending data to the base station. A UE has performed measurement on some cell, i.e., obtaining downlink synchronization with the cell; otherwise, the procedure similar to cell searching needs to be carried out. The uplink synchronization is obtained by carrying out a random access procedure (the Time Advance (TA in short) of sending is obtained simultaneously), the main usage of TA is for UE to determine the moment of data sending; after UE obtaining uplink synchronization, the time Alignment Timer (TAT in short) of uplink synchronization is started; if the UE receives the TA sent from the base station to the UE before the TAT expires, it will be deemed that the UE keeps uplink synchronization with the base station; otherwise, the TAT expires, the UE is deemed to lose uplink synchronization, and the terminal only releases relevant resources and notifies the PRC layer. After the terminal loses the uplink synchronization, if it still needs to send data to the base station, the uplink synchronization needs to be re-obtained. Since the cells only have one carrier in the LTE system, there is only one TA. The UE will carry out a random access procedure under the following several conditions: initial access from IDLE state, RRC connection re-establishment, handoff, DL (Downlink) data arrival, UL (Uplink) data arrival, and location service, wherein the handoff and the DL data arrival are the random access procedures which need to be carried out by receiving indication requirement from the base station. In addition, the terminal will perform radio link failure (RLF) monitoring on the serving cell; if occurrence of the RLF is detected in the serving cell, the RRC connection re-establishment will be triggered; the cell selection procedure is first carried out, then the random access procedure is carried out on the selected cell.
After the introduction of the carrier aggregation (CA in short) technique, the UE can simultaneously communicate with the source base station through multiple component carriers (such as CC1, CC2) after entering the connection state, and the Primary Cell (Pcell) and the Secondary Cell (Scell) are introduced. Due to the increase of the amount of data, the number of Scells will be increased, such as being increased to 4, the scene will also be relaxed, such as supporting uplink RRH and repeater; one TA cannot solve the problem, so multiple TAs will be introduced. For the ease of management, the serving cells using the same TA are classified as one TA group, the TA group including the Pcell is called PTAG, the TA group not including the Pcell is called STAG. When the UE is on the PTAG, the timing for the Pcell to perform random access is the same as that in the case of one TA; When the UE is on the STAG, the timing for the cell to perform random access is only one, that is, a network side notification. Further, the RLF (Radio link failure) monitoring is only conducted on the Primary Cell; if occurrence of the RLF is detected in the serving cell, the RRC connection re-establishment will be triggered. The Secondary Cell does not perform the RLF monitoring.
Due to the deficiency of spectrum resource, and the sharp increase of the traffic with large flow of the mobile subscriber, the demand for using high frequency point, such as 3.5 GHz, to conduct hotspot coverage is increasingly apparent, and using lower-power nodes becomes a new application scene with the object of increasing the throughput of the users and enhancing the mobility performance. Due to the considerable signal attenuation of the high frequency point, the coverage range of a small cell is relatively small, without common sites with the existing cells, many companies and operators currently tend to seek a new enhancing solution, and the Dual-Connection is one of them. The terminal in the Dual-Connection can maintain data connection with more than two network nodes at the same time, and the control connection only exists in one network node. Regarding how to trigger the random access procedure of a small cell, there is no technical solution disclosed yet.